Method and apparatus for removing high concentration acid gas from natural gas

ABSTRACT

A multi-stage process for recovering acid gas from natural gas having high acid gas contents utilizes two or more membrane absorption contactors arranged in series. The first membrane absorption contactor uses a physical solvent to remove a high volume of acid gas transferred across a membrane, and to reduce the acid gas content in the natural gas to a lower level that can be managed using chemical solvents. The second and, if needed, subsequent membrane absorption contactors can use a chemical solvent to remove acid gas transferred across the respective membranes and reduce the acid gas content in the natural gas to very low levels, if needed, depending on product specifications.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of application, U.S. Ser.No. 14/277,255, filed on 14 May 2014. The co-pending parent applicationis hereby incorporated by reference herein and is made a part hereof,including but not limited to those portions which specifically appearhereinafter.

FIELD OF THE INVENTION

This invention is directed to a multi-stage method and apparatus forremoving an acid gas present in high concentrations in natural gas.

BACKGROUND OF THE INVENTION

Untreated natural gas can contain high concentrations, even majorityconcentrations of up to 80% by volume of acid gas. The acid gas iscomposed mainly or entirely of carbon dioxide, but can also includehydrogen sulfide, sulfur dioxide, carbon disulfide, hydrogen cyanide andcarbonyl sulfide. Such high amounts of carbon dioxide and lesser amountsof other acid gases are unsuitable and unsafe for natural gas used inresidential and industrial applications.

Processes for removing acid gases from natural gas typically includemembrane separators in which the natural gas is fed to one side of amembrane separator and the acid gas component is caused to diffusethrough the membrane, separating it from the natural gas. Theseprocesses have drawbacks when the acid gas is present at highconcentrations, due to the large size of the membrane separator requiredfor effective treatment. Also, high concentrations of acid gas such ascarbon dioxide can plasticize the polymeric separator membranes andreduce their separation efficiency. Also, these processes cannot achievehigh removal effectiveness to achieve product gas quality in many cases.

There is a need or desire for a method and apparatus for removing highconcentration acid gases from natural gas, which addresses the foregoingsize, quality, and durability issues.

SUMMARY OF THE INVENTION

The present invention is directed to a multi-stage method and apparatusfor removing an acid gas present in high concentrations in natural gas.The invention combines the advantages of physical solvents and chemicalsolvents in different stages of membrane separation to provide asolution that is cost-effective and durable. A physical solvent isemployed in a first stage to reduce the concentration of acid gas from afirst high level to a second lower level that can be more easily managedusing a chemical solvent without requiring excessive size and expense. Achemical solvent is employed in second and, if needed, subsequent stagesto reduce the concentration of acid gas to still lower levels that meetproduct specifications.

The method includes the steps of supplying natural gas having a firstconcentration of acid gas to a first side of a first membrane, andsupplying a physical solvent to a second side of the first membrane,suitably in a first membrane absorption contactor. Acid gas isselectively transferred from the natural gas through the first membranefrom the first side to the second side, yielding natural gas having asecond (lower) concentration of acid gas on the first side of the firstmembrane. The natural gas having the second concentration of acid gas isthen supplied to the first side of a second membrane, suitably in asecond membrane absorption contactor, and a chemical solvent is suppliedto the second side of the second membrane. Acid gas is selectivelytransferred from the natural gas through the second membrane, from thefirst side to the second side of the second membrane, yielding naturalgas having a third (lower) concentration of acid gas on the first sideof the second membrane. The natural gas having the third concentrationof acid gas is then recovered from the first side of the secondmembrane. The process can be continued in subsequent stages, as needed,until the natural gas reaches or falls below a specified concentrationof acid gas.

With the foregoing in mind, it is a feature and advantage of theinvention to provide a method and corresponding apparatus that reduceshigh concentrations of acid gas in natural gas to product specificationlevels, without incurring expensive solvent and equipment costs, spacerequirements, or process durability issues. These and other features andadvantages will become further apparent from the following detaileddescription of the invention read in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a multi-stage process of the invention,shown as including three stages, for removing acid gas from natural gas.

FIG. 2 is a cutaway view of a membrane absorption contactor 20 shown inFIG. 1, and is also representative of membrane absorption contactors 30and 40 shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a multi-stage process 10 for removing acidgas from natural gas is illustrated. Acid gases found in natural gasinclude, for example, carbon dioxide, hydrogen sulfide, sulfur dioxide,carbon disulfide, hydrogen cyanide and carbonyl sulfate. Acid gases,particularly carbon dioxide, can be present at up to 80% by volume ofnatural gas extracted from geological sources. The process 10 isdesigned to reduce the acid contents to commercially acceptable productlevels (typically below 2% by volume for pipeline gas or below 50 ppmvfor LNG specification gas), and is especially suitable for natural gashaving initial acid gas concentrations of at least about 20% by volume,or at least about 35% by volume, or at least about 50% by volume, and upto about 80% by volume. The multi-stage process 10 is designed to reducethe acid gas concentration in the natural gas to not more than about 10%by volume in its first stage, not more than about 2% by volume in itssecond stage, and not more than about 0.1% by volume, suitably not morethan about 50 ppm by volume in its third and/or final stage. While theselimits apply to all acid gases combined, carbon dioxide is typically theprimary acid gas component. In situations where the acid gas includeshydrogen sulfide in addition to carbon dioxide, the multi-stage process10 is also designed to reduce the hydrogen sulfide content in thenatural gas to not more than about 4 ppmv in the second stage or, ifneeded, in the third and/or final stage.

Natural gas 12 having a first (high) concentration of acid gas is fedfrom a source 14 which can be a natural gas well or pipeline leadingfrom a point of origin. The supply pressure and flow rate of the naturalgas 12 can be regulated by valve 15. If necessary, the natural gas 12can be cooled using cooler 18. The natural gas 12 is then supplied to aninlet 19 of a first stage membrane absorption contactor 20.

The first stage membrane absorption contactor 20 can include a shellside 22 and a bore side 24 separated by a membrane 26 as shown in FIG.2. The membrane 26 can be disposed in any suitable manner that permitsthe diffusion of acid gas through the membrane yet blocks the flow ofnatural gas. In the embodiment shown in FIG. 2, the membrane 26surrounds a bore side 24 which can be defined by a rigid porous supportplate or screen, or any suitable porous material shown at 25. Membrane26 is suitably a non-porous material that selectively diffuses the acidgas from the first side 26A to the second side 26B of the membrane, thustransferring the acid gas from the shell side 22 to the bore side 24 ofthe membrane absorption contactor 20 while maintaining the natural gason the first side 26A of the membrane and the shell side 22 of themembrane absorption contactor. Alternatively, the membrane 26 can be amicroporous hydrophobic material with pores small enough to selectivelypermit the transfer of acid gas from natural gas into the physical orchemical solvent but not the transfer of solvent into the natural gas.

Non-porous membranes are films made of polymers having amorphoussegments that selectively allow acid gases to pass through by a solutiondiffusion mechanism. The selectivity and transfer rate of non-porousmembranes are significantly affected by temperature. Suitable non-porousmembrane polymers include without limitation polyvinylidene fluoride,polypropylene, cellulose acetate, polysulfone, polycarbonate, polyimide,polyamide, etc., and combinations thereof.

Microporous hydrophobic membranes have voids connected by pores whosediameters are large enough to facilitate the transfer of acid gases butsmall enough to block the transfer of solvent into natural gas. Suitablemicroporous membrane polymers include without limitation polypropylene,polyethylene, polyperfluoroalkoxy, polyetheretherketone,polytetrafluoroethylene, polyvinylidene fluoride, etc., and combinationsthereof.

A physical solvent is supplied to the second side 26B of the membrane 26through an inlet 21 that passes into the bore side 24 of first stagemembrane absorption contactor 20. The physical solvent originates fromsource 11, which can be a storage tank, and can be pumped using pump 16and warmed or cooled using heat exchanger 13. A physical solvent is asolvent that relies on physical solubility, as opposed to chemicalreaction, to dissolve the acid gases. Suitable physical solvents includewithout limitation dimethyl ethers of polyethylene glycol, methanol,N-methyl-2-pyrrolidone, propylene carbonate, water, diethylene glycol,silicone fluid, aliphatic and aromatic hydrocarbons, alcohols, ketones,aldehydes, N-formyl morpholine, N-acetyl morpholine, etc., andcombinations thereof. Because solubility decreases with increasingtemperature, it is generally advantageous to cool the physical solventusing heat exchanger 13.

As the acid gas from the natural gas 12 is transferred from the firstside 26A to the second side 26B of membrane 26, it dissolves in thephysical solvent and is transported (along with the physical solvent)through outlet 29 of membrane absorption contactor 20. The selectivetransfer of acid gas through the membrane 26 yields natural gas having asecond (lower) concentration of acid gas on the first side 26A of themembrane 26, the second concentration typically being not more thanabout 10% by volume. After the physical solvent exits the membraneabsorption contactor 20 through bore side outlet 29, it can be heated toreduce the solubility of the acid gas and cause substantial separation,flashed at a lower pressure to release the acid gas and causesubstantial separation, or heated and flashed to release acid gas. Thephysical solvent can then be recycled for further use in the membraneabsorption contactor 20.

Alternatively, the acid gas containing natural gas can be fed to thebore side 21 of the membrane absorption contactor 20 and the solvent fedto the shell side 19 of the membrane absorption contactor 20. The sweetnatural gas can then be recovered from the bore side outlet 29 and theacid gas laden solvent recovered from the shell side outlet 27.

The natural gas having the second concentration of acid gas exitsmembrane absorption contactor 20 via shell side outlet 27 and issupplied via transfer line 32 and heat exchanger 28 (if needed) to theshell side inlet 19 of the second stage membrane absorption contactor30. The second membrane absorption contactor 30 can be configured in thesame or similar fashion as the first membrane absorption contactor 20and is therefore described with like reference numerals as shown in FIG.2. The second membrane absorption contactor 30 includes a shell side 22,a bore side 24, a second membrane 26 having a first side 26A and asecond side 26B and supported by a porous support or screen 25, a shellside inlet 19 and outlet 27, and a bore side inlet 23 and outlet 29.

The natural gas having the second concentration of acid gas is suppliedto the first side 26A of the second membrane 26. A chemical solvent fromsource 31, which can be a holding tank, is supplied via pump 17 and heatexchanger 23 and bore side inlet 21 to the second side 26B of the secondmembrane. A chemical solvent is one that chemically reacts with the acidgas to effect dissolution. Suitable chemical solvents include withoutlimitation aqueous ethanolamine solutions, aqueous potassium, sodium, orammonium carbonate solutions, other alkaline salt solutions, ionicliquids, ammonia, aqueous diglycolamine solutions, triazine solutionsetc., and combinations thereof.

The second stage membrane 26 may be non-porous or micro-porous asdescribed above. Acid gas present in the natural gas having the secondconcentration is transferred through the second membrane 26, from thefirst side 26A to the second side 26B, yielding natural gas having athird (lower) concentration of acid gas on the first side 26A of secondmembrane 26. The third concentration of acid gas is typically not morethan about 2% by volume, which is a common pipeline specification fornatural gas. The natural gas having the third concentration of acid gasis recovered from the shell side outlet 27 of second stage membraneabsorption contactor 30. The chemical solvent exits through bore sideoutlet 29 and can be separated from the acid gas and recycled usingconventional methods of heating and stripping.

Because the first and second stage membrane absorption contactors 20 and30 rely on the transfer of acid gas from the first side 26A to thesecond side 26B of a corresponding membrane 26, it may be advantageousto provide a pressure differential across the membrane to facilitatesuch transfer. While the natural gas 12 provided from the source 14 mayhave a very high initial pressure of up to about 80 atmospheres, or upto 100 atmospheres or more, the pressure differential across themembranes 26 should not be so high as to damage the membranes 26. Themembrane absorption contactors 20 and 30 can operate without a pressuredifferential if it is advantageous to operate the membrane absorptioncontactors in such a manner.

The natural gas having the third concentration is then carried viatransfer line 42 and heat exchanger 38 (if needed) to the third stagemembrane absorption contactor 40 which can be configured in the same orsimilar fashion as the first and second stage membrane absorptioncontactors 20 and 30 and can be described using the same referencenumerals in FIG. 2. The natural gas having the third concentration ofacid gas is supplied via shell side inlet 19 to the first side 26A ofthird membrane 26 in third stage membrane absorption contactor 40,suitably under modest pressure as described above. A chemical solvent,which can be the same or different chemical solvent used in the secondmembrane absorption contactor 30, is supplied from a source 41 via pump60 and heat exchanger 33 (if needed) and through the bore side inlet 21,to the second side 26B of third membrane 26.

Acid gas from the natural gas having the third concentration istransferred through the third membrane 26, from the first side 26A tothe second side 26B of the third membrane 26, yielding natural gashaving a fourth (lower) concentration of acid gas on the first side 26Aof third membrane 26. The fourth concentration can be low enough to meetspecifications for liquid natural gas and is suitably not more thanabout 50 ppm by volume. The natural gas having the fourth concentrationof acid gas can then be recovered from the first side 26A of thirdmembrane 26 via the outlet 27 and flow regulator 59. The chemicalsolvent can be recovered from the third membrane absorption contactor 40via outlet 29, and can be separated from the acid gas using knowntechniques and recycled.

While the invention is described in three membrane absorption contactorstages, the invention can be practiced using two, four or more membraneabsorption contactor stages depending on the amounts and types of acidspresent in the incoming natural gas and the specifications for theproduct natural gas, as well as the natural gas flow rates and otherprocess conditions. The invention saves money and space by combining thecost advantages of physical solvents with the performance advantages ofchemical solvents to absorb acid gases through the respective membranes26 and take the acid gas concentrations in natural gas from very high tovery low levels.

The embodiments of the invention described herein are presentlypreferred. Various modifications and improvements can be made withoutdeparting from the spirit and scope of the invention. The scope of theinvention is defined by the appended claims and includes all changesthat fall within the meaning and range of equivalents.

We claim:
 1. An apparatus for removing acid gas from natural gas,comprising: a first stage membrane absorption contactor including afirst membrane having a first side and a second side; a second stagemembrane absorption contactor including a second membrane having a firstside and a second side; a first supply line providing natural gas havinga first concentration of acid gas to the first side of the firstmembrane; a second supply line leading from a source of physical solventto the second side of the first membrane; a first transfer mechanism forselectively transferring acid gas from the first side to the second sideof the first membrane, lowering the concentration of acid gas in thenatural gas on the first side of the first membrane to a secondconcentration; a first transfer line leading from the first side of thefirst membrane to the first side of the second membrane for supplyingthe natural gas having the second concentration of acid gas to the firstside of the second membrane; and a second transfer mechanism forselectively transferring acid gas from the first side to the second sideof the second membrane, lowering the concentration of acid gas in thenatural gas on the first side of the second membrane to a thirdconcentration.
 2. The apparatus of claim 1, further comprising a thirdsupply line leading from a source of chemical solvent to the second sideof the second membrane.
 3. The apparatus of claim 1, wherein the firsttransfer mechanism comprises a mechanism for providing a first pressuredifferential across the first membrane and the second transfer mechanismcomprises a mechanism for providing a second pressure differentialacross the second membrane.
 4. The apparatus of claim 1, furthercomprising: a third stage membrane absorption contactor including athird membrane having a first side and a second side; a second transferline leading from the first side of the second membrane to the firstside of the third membrane for supplying natural gas having the thirdconcentration of acid gas to the first side of the third membrane; and athird transfer mechanism for selectively transferring acid gas from thefirst side to the second side of the third membrane, lowering theconcentration of acid gas in the natural gas on the first side of thethird membrane to a fourth concentration.
 5. The apparatus of claim 4,further comprising a fourth supply line leading from a source ofchemical solvent to the second side of the third membrane.
 6. Theapparatus of claim 4, wherein the third transfer mechanism comprises amechanism for providing a third pressure differential across the thirdmembrane.